Microemulsions have attracted interest for use in many applications due to their ability to improve solubility, their phase stability, their increased shelf life compared to macroemulsions, and their ease of preparation, not requiring high-energy mixing techniques.
An emulsion comprises a first phase that is liquid under conditions of use, at least one additional phase that is liquid under conditions of use and immiscible with the first liquid phase, and a surfactant. Each liquid phase may comprise more than one ingredient and other ingredients may optionally be present. One of the liquid phases often comprises water, but this is not a requirement. In addition, a microemulsion may comprise a coupling agent. When properly selected and present in a favorable concentration ratio, these components spontaneously emulsify to form a, thermodynamically stable and visually transparent microemulsion. In contrast, normal emulsions (macroemulsions) typically require energetic mixing to form and are opaque and thermodynamically unstable, separating over time into layers. A properly composed microemulsion concentrate, comprising all ingredients of a microemulsion except for one of the liquid phases, can be added to that one liquid phase and will form a microemulsion with only gentle mixing being required.
Selection and design of microemulsion compositions is complicated, time-consuming, and unpredictable. Although numerous microemulsion systems are known, a surfactant package (surfactant and coupling agent) that is effective for one pair of immiscible liquids will not necessarily be effective for a different pair of immiscible liquids.